Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EST Mon Nov 5 2018

Valid 00Z Tue Nov 6 2018 - 00Z Fri Nov 9 2018

...Northern and Central Rockies...

Multiple mid level perturbations within northwest flow aloft
through Wednesday are expected to produce widespread mountain snow
from the Bitterroots of central-northern Idaho, to the Big Horn
mountains of central Wyoming, with lesser amounts for the
Cascades.  On day 1 (Monday night and into Tuesday) the heaviest
snow is expected for the ranges of Idaho and far western Montana. 
WPC probabilities are moderate for 8 inches or more of snow for
the Bitterroot mountains.

For the Day 2 period (Tuesday night into Wednesday), another
shortwave impulse tracks towards the southeast and enhances lift
and increases low level RH, along with a strong upper jet and
low-mid level frontogenesis in place to aid in ascent and produce
another round of moderate snow.  The highest accumulations are
favored for the high terrain of the Bitterroots, and ranges of
western Montana and northwest Wyoming, where WPC probabilities
feature a moderate to high risk for 4+ inches.  A drier weather
pattern returns by Thursday for this region with just a few
lingering snow showers remaining for the Absarokas and Big Horn
mountains.


...Minnesota to the northern Great Lakes...

Surface cyclogenesis across the Upper Midwest and the northern
Great Lakes region, combined with a mid-level impulse acquiring
negative tilt and increasing low-mid level RH, is expected to
produce snow showers to the northwest of the surface low.  This
will mainly affect parts of northern Wisconsin and the western
Upper Peninsula of Michigan, where 1 to 3 inches of snow appears
likely.  Northwesterly flow across Lake Superior will provide some
lake enhancement as steeper lapse rates develop owing to cold air
advection.  Lee shore convergence will also provide lift over this
region, and WPC probabilities are slight for 4+ inches.


...Central plains on Day 3...

A mid level impulse tracking southeast around the base of the
large scale trough over the north-central U.S. is expected to
produce a band of accumulating snow from eastern Colorado to
northern Kansas.  Although no well-defined surface low is expected
with this, there should be enough mid-upper level forcing to
support a 6 to 12 hour period of light to moderate snow.  Easterly
low level flow will result in an upslope component and further aid
with lift.  Snowfall rates may be locally enhanced owing to some
elevated instability and steeper lapse rates, promoting more
vigorous lift in the dendritic growth zone.  There may be a brief
period of sleet at the onset for some areas.

There are multiple EC and GEFS ensemble members that are
indicating the potential for 4 inches or more of snowfall, with
the greatest consensus for this across northwest Kansas.  WPC
probabilities are low for 4+ inches of snow during the Day 3
period Thursday.  


The probability of significant icing is less than 10 percent days
1-3 nationwide.

D. Hamrick